Method of employing a high percentage of reground thermoplastic scrap resin in an extruder

ABSTRACT

Method of employing a high percentage of reground polyethylene resin scrap in an extruder, wherein fluffy, low-density, polyethylene scrap resin is preferably admixed with a granular high-density polyethylene virgin resin, and the admixture fed by gravity into a first feed inlet in a screw-type conveyor, and granular polyethylene virgin resin fed by gravity into a second feed inlet generally adjacent to and downstream or the first feed inlet of the extruder. The admixture fed to the first feed inlet is insufficient to fill the volume of the extruder tube, while the virgin resin automatically fed by gravity fills up the total volume in the extruder tube, thereby preventing starvation of the resin to the extruder die in the event of blockage of the fluffy scrap resin, which technique permits high amounts of from 20 to 40% of scrap resin to be blended with virgin resin.

REFERENCE TO PRIOR APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 237,258,filed Mar. 23, 1972 now abandoned.

BACKGROUND OF THE INVENTION

The employment of scrap thermoplastic resins in an extrusion process isa common method of efficiently and profitably recycling scrap resin.Typically, the scrap resin material is ground up and then fed into ascrew-type extruder through a feed hopper, and often in admixture with avirgin resin material. By such recovery and recycle techniques,scrap-to-virgin resin ratios above to about 20% or higher have beenreported; however and typically, the scrap-to-virgin resin ratiosusually obtained have been up to about 10%, and more generally 5%.

One prime difficulty in the recycle of scrap polyethylene resin, or asimilar resin, is that, on regrinding of the solid resin, the resinbecomes a very low-density, fine particle-size, fluffy material. Thereground resin is thus difficult to feed in a feed hopper in acontinuous manner to an extruder, and, further, is difficult, by virtueof the difference in density, to admix properly with a granular virginresin for a proper and continuous feed to an extruder. Thus, when thefluffy scrap resin, alone or with virgin resin, is attempted to be fedat high ratios into an extruder, quite often blockage of the feed hopperoccurs, with the result that the extruder die is then starved for resinmaterial. Such difficulties have often resulted in expensive down timefor the extruder, while restricting the practical amount of the scrapresin to be employed in a recycling process to not more than 5%. Thus,there exists a need for an apparatus and method by which thermoplasticscrap resin material, such as polyethylene, may be effectively andefficiently employed in an extruder at high ratios, without thedifficulties occasioned by the prior art.

SUMMARY OF THE INVENTION

My invention relates to an apparatus for and method of efficiently andprofitably employing scrap thermoplastic resin in an extruder. Inparticular, my invention relates to an apparatus for and a method ofemploying a high percentage of a thermoplastic scrap resin, such asreground polyethylene scrap resin, with additional virgin thermoplasticresin, such as virgin polyethylene scrap resin, in a screw-typeextruder.

My invention provides for the profitable and effective utilization ofscrap resin at much higher ratios than that achieved by the prior art,and, for example, greater than 10% and typically from 20 to 40% orhigher. My discovery overcomes many of the disadvantages of the priorart in the use and handling reground scrap resin, while preventing thestarvation of an extruder die of an extruder by the blockage of feed ofthe low-density, fluffy, reground resin scrap. In my invention, theblockage of the feed of the fluffy scrap resin, alone or in admixturewith virgin resin granules, does not result in the starvation of theextruder die of the extruder, which, in the past, has limitedconventional extruders and recycle techniques through the use of verylow ratios of scrap-to-virgin resin in practice.

My apparatus includes a means to regrind a solid scrap resin material,such as an olefinic resin like polypropylene or polyethylene resin, intothe form of a fine particle-size material characterized by very lowdensity and a fluffy snow-like nature. In my invention, the regroundscrap resin may be used alone, or preferably admixed with an additionalvirgin thermoplastic resin of the same or different resin from the sameor different source. Preferably, the virgin resin is the same or similarin composition, and derived from a common source, such as the same orsimilar extruding machine or process. The reground resin or admixture isthen conveyed, such as by a blower, into a feed hopper or other storagebin, preferably a cyclone-type truncated hopper. The reground scrapresin or a combination of the scrap resin and virgin resin material isthen fed from the feed hopper by gravity or a low-pressure differenceinto a first feed inlet into the tube of a single or multiple screwextruder. The extruder typically comprises an extruder barrel having arotating lead screw housed therein to form a mixing and compressingsection, and later a heated extrusion section wherein the resin isheated to a molten stage, and then through an extrusion die wherein theheated or molten resin is extruded into a product, such as a blowntubular form in one embodiment, to be slit subsequently and formed intoa thermoplastic thin-film roll. In the extruder, rotation of the leadscrew therein in the mixing and compressing sections compresses theplastic material and propels it forward. The fluffy scrap resin materialoften has a tendency, because of its low density and resistance to flow,to become compressed and blocked in the feed hopper throat, therebystarving the extruder die for resin material. Although this may happeninfrequently in the prior art, such blockages are annoying,time-consuming and expensive.

The scrap resin or admixture fed to the barrel of the extruder is insuch a quantity as to be insufficient to fill completely the volume ofthe lead screw tube, and typically fills about 40 to 70% of the leadscrew. Of course, if too much resin is fed into the lead screw; that is,greater than the output of the extruder die, then backup of the scrapresin or admixture occurs in the feed hopper.

My apparatus contains a second or auxiliary feed hopper adapted tocontain a granular thermoplastic resin material, such as a medium orlow-density virgin polyethylene resin in pellet or granular form, buttypically of the same or similar-type resin as that admixed with thefluffy scrap resin after regrinding. At a further distance generallyadjacent to and quite close to the first feed inlet along the barrel,there is provided a second feed inlet for the introduction, typically bygravity, of the virgin resin material from the second feed hopper intothe extruder. The granular virgin resin material from the auxiliaryhopper is fed into the second feed inlet to fill all or substantiallyall of the volume of the barrel. The relationship and position of thefirst and second feed inlets to the mixing and compressing sections ofthe extruder is such that the second feed inlet is positioned at leastone helical turn distance, and often nor more than three helical turnsdistance, from the first feed inlet. The fluffy snow-like resin from thefirst feed hopper fed into the lead screw is quickly compressed andreduced in volume by one helical turn of the screw, permitting the feedof additional fluffy scrap resin from the first hopper, and permittingthe easy introduction by gravity of the virgin resin material from thesecond feed hopper in the second feed inlet. Due to the low density andfluffy characteristics of the scrap resin or admixture, the opening ofthe first feed inlet is preferably larger, such as from two to fourtimes, than the area of the second feed inlet, wherein easy-to-handle,colored or granular-type virgin plastic material of high density isintroduced into the lead screw. The size of the respective openings andthe positions thereof, therefore, permit the proper feed of the scrapand virgin material, while permitting a void in the volume of the barrelbetween the first and second feed inlets sufficient to permit theautomatic feed by gravity of the virgin material into the extruder.Further and importantly, the first feed hopper is preferably a cycloneor conical-type hopper so that the fluffy scrap material is urged bygravity and the low pressure in the feed throat easily into the extrudertube. The scrap and virgin material which may be same or of a slightlydifferent size, for example, ranging from 1/16th of an inch to 1/4th ofan inch in average particle size, is of considerable; e.g., 3 to 10times, difference in density.

The entire admixture of the scrap and the virgin resin material sointroduced into the lead screw is fed by the rotation of the lead screwafter mixing and compressing into a heating section of the extruderbarrel, where the compressed and mixed blend is heated to a temperaturefor extrusion, and then subsequently into an extrusion die wherein theheated material is extruded into the desired form.

In the event that the first feed hopper containing a scrap resinmaterial becomes temporarily blocked due to the lack of sufficientdensity of the scrap material, my discovery permits the extrusion die tocontinue in full production, since the amount of virgin resin fed fromthe second feed hopper into the extruder will increase automatically tofill the void in the feed tube caused by the blockage; thus, permittingthe continuous operation of the extruder. The virgin resin fed from thesecond or auxiliary feed hopper, therefore, makes up in quantity for theamount of scrap and/or virgin resin which has been temporarily blockedfrom feeding into the extruder from the first hopper. By my apparatusand method, the costly starveouts of the extruder die and productionshutdowns are prevented, since the use of high scrap content in the feedinlet of conventional extruder machines increases the likelihood of suchresin starveouts of the extruder die, and conventional extruders andscrap-recycling techniques are limited to low scrap ratios of the virginmaterial.

In my invention, the salvagable resin may be fed into the grinding meansin any form suitable for grinding, while the admixture of the granularvirgin resin with the low-density reground resin may vary as desired,but typically ranges in a scrap-to-virgin resin ratio of from about 1:1to 6:1, but particularly, from about 2-4:1, such that the amount ofscrap material in the final extruded product is greater that 20% or ashigh as tolerable, depending on the extrusion speed, resin and otherfactors.

My apparatus and method will be described for the purposes ofillustration only in connection with the employment of a polyethylenescrap resin and virgin resin admixtures in a single screw-type extruder.However, as will be apparent to those skilled in the art, various andobvious changes may be made in the various embodiments of the inventionas illustrated, and it is not intended that the matter contained hereinand as described in the specific embodiments be so limited in scope.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention and the objects and features thereof may be understood withreference to the following detailed description of an illustrativeembodiment of my apparatus in the drawings in which:

FIG. 1A is a partial diagrammatic sketch of my apparatus for thegrinding of scrap resin and the feeding of scrap resin and a virginresin admixture into an extrusion die in accordance with my invention;and

FIG. 1B is a partial diagrammatic sketch of the remainder of thediagrammatic sketch of FIG. 1A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A and FIG. 1B illustrate an apparatus in which rolls 11 ofpolyethylene resin material to be salvaged are fed by feed rolls 12 intogrinder 13 wherein the sheet material is ground into a low-density, fineparticle-size fluff material; e.g., about 1/8th of an inch in size. Thereground polyethylene material 33 flows into exit tube 24 where it ismixed with hard, high-density, pellet or granular virgin plasticmaterial 15, fed into hopper 16, and into exit tube 24. The ratio ofvirgin material to scrap material at this stage is preferably in theorder of one part of virgin material to 3.5 to 4.5; e.g., 4, parts ofscrap ground material, forming the mixture 14 in tube 24.

Blower unit 17 draws in the mixture of material 14 and ejects it throughexhaust tube 18 into hopper 19 where it flows by gravity to the hopperthroat and feed inlet 26 leading into the extruder barrel 20. Feed screw21, rotating in barrel 20, is rotated by motor unit 22. The feed screw21 acts to mix, compress and force the material in tube 20 to theextruder heater and die section 35 as shown in FIG. 1B.

Pellet virgin polyethylene resin; e.g., a low or medium-density resinlike the resin 15, is added to auxiliary feed hopper 23 where it flowsby gravity through tube 31 to throat 25 joining barrel 20 to mix withthe initial mixture 14 of scrap and virgin material in the tube, formingmixture 34.

Under normal operation, the ratio of virgin material added by theauxiliary hopper 23 to the mixture 14 in the barrel 20 is in the ratioof one part of virgin material 15 to one part of mixture 14, resultingin a mixture 34 which is composed of about six parts of virgin materialto four parts of reground scrap material. Mixture 34 is fed throughbarrel 20 by feed screw 21 to extrusion die unit 35, mounted on supportunit 37, where the material 34 is extruded into plastic products 36.

As illustrated, the feed hopper 19 comprises a cyclone or conical-typehopper having tapered sides leading to the first feed inlet or hopperthroat 26, where by virtue of the blowing of the scrap resin into thefeed hopper, there is a pressure difference which induces movement ofthe scrap admixture 14 from the hopper 19 through the flute 26 and intothe barrel 20. As illustrated, the feed hopper 19 is positioned suchthat the throat communicates directly with the feed inlet, while thefirst feed inlet and throat of the hopper 26 is upstream of and adjacentto the second inlet from the gravity feed of hopper 23 for the virginmaterial 15. Typically, the throat section 26 would have an area two tosix times greater than the second feed inlet area 25 in order to permitpassage of the fluffy scrap admixture into the barrel 20. In oneillustrated embodiment, the hopper 26 would comprise a generallyrectangular opening of about 6 by 4 inches, while the second feed inletor throat 25 would comprise a circular inlet of about 3 to 4 inches indiameter, with the distance between the centers of the first and secondthroats in a single extruder being about 6 inches.

It is not unusual for the fine fluff of the reground scrap material inhopper 19 to separate from the heavier virgin material in the hopper 19and to block up the throat of the hopper, causing the incoming materialin tube 18 to back up and flow out of the hopper top 32. In conventionalapparatus, such blockage would prevent sufficient material from flowinginto the barrel 20 to force properly sufficient material through theextrusion die unit 35, a condition known as extruder starveout. Such acondition is very costly because the material in the extruder die areabecomes overheated and must be scrapped, resulting in expensive downtime for the machine.

With the apparatus of my invention, a blockage of adequate flow fromhopper 19 of mixture 14 of scrap and virgin plastic is compensated forby an automatic increase of flow of virgin pellets 15 from auxiliaryhopper 23 feeding through throat 25 into barrel 20. This increased flowof virgin material 15 occurs since the barrel 20 is fed automatically bygravity from hopper throat 25, with the limit on the feed of virginmaterial 15 from hopper 23 being the actual filling of volume of thebarrel 20 under throat 25.

The difference in positions between the throats 25 and 26 as illustratedof one helical turn permits the fluffy resin admixture 14 to becompressed and reduced in volume prior to the introduction of the virginresin 15. It is essential in my invention that the fluffy scrap resin beadded to the first feed inlet and compressed at least in part by thelead screw prior to the introduction of the virgin resin.

The blockage of the scrap mixture 14 in hopper 19 does not causeextruder starveout, and the extrusion die 35 continues in operationduring such a blockage in hopper 19. Blockage in hopper 19 may becleared by temporarily stopping blower 17 and allowing the material inhopper 19 to settle together or separately with necessary manualagitation of the contents of hopper 19. After the blockage has beencleared, the grinder unit 13 and the blower unit 17 may be restarted toresume the original feeding schedule. During this interruption, theextrusion process continues, with the only change during the blockagebeing the temporarily increased ratio of virgin material to scrapmaterial utilized in the finished product.

What is claimed is:
 1. A process for the use of high amounts of afluffy, low-density, scrap resin in an extruder, which extrudercomprises a barrel with a rotating helical lead screw therein and anextruder die at one end thereof, and characterized by a first feed inletand a second feed inlet downstream of the first feed inlet, whichprocess consisting essentially of:a. introducing into the first feedinlet of the barrel a fluffy, thermoplastic, scrap resin which tendsoccasionally to plug the first feed inlet; b. moving the fluffy scrapresin in the barrel downstream to the second feed inlet by the rotatinghelical movement of at least one helical screw length of the lead screw;c. introducing continuously and automatically by gravity feed into thesecond feed inlet and into the unoccupied volume of the extruder barrela granular, thermoplastic, virgin resin in pellet form in an amountsufficient to keep the extruder barrel substantially full of resin atall times during the extruder operation when the fluffy resin plugs thefirst feed inlet, so that, in the event of any blockage of the fluffyresin in the first feed inlet, resin starvation of the extruder die isprevented by the introduction of needed high-density resin into thesecond feed inlet of the barrel; and d. extruding the scrap-virginthermoplastic resin admixture.
 2. A process for using fluffythermoplastic scrap as extruder feed consisting essentially of:a.grinding thermoplastic resin scrap into a fine fluffy material of acharacter having a tendency to block an extruder barrel feed inlet; b.feeding said material directly into an extruder barrel first feed inletin an amount insufficient to fill the extruder barrel; c. feeding asecond thermoplastic resin in pellet form into a second feed inlet inthe extruder barrel, said second feed inlet being downstream from thefirst inlet and spaced therefrom by at least one helical screw length,said pellets being introduced automatically by gravity feed in an amountsufficient to keep the extruder barrel substantially full of resin atall times; and d. extruding the scrap resin and second resin as amixture through an extruder die, whereby starvation of the extruder dieis prevented in the event of plugging of the first extruder inlet. 3.The process of claim 2 wherein the thermoplastic resin is an olefinicresin.
 4. The process of claim 2 wherein the thermoplastic resin is apolyethylene resin.
 5. The process of claim 2 wherein the scrap resin isderived from the operation of a film extruder.
 6. The process of claim 2which includes the step of mixing the ground fluffy resin scrap with agranular thermoplastic resin of higher density to obtain an admixture ina ratio of scrap-to-virgin resin of from about 1:1 to 6:1, and feedingthe admixture into the first feed inlet.
 7. The process of claim 2wherein the amount of granular resin and fluffy scrap resin employed inthe process provides for an extruded product when the first feed inletis not plugged by scrap resin comprising from about 20 to 40% by weightof scrap resin.
 8. The process of claim 2 wherein the scrap resin isadmixed with high-density virgin resin and, after being fed into thefeed inlet and compressed in the barrel, comprises an admixture of scrapresin and virgin resin, which occupies at least one-half of the volumeof the barrel of the extruder.
 9. The process of claim 2 which includesthe step of extruding the mixture through an extrusion die into a thinblown tube product.
 10. The process of claim 2 which includes blowingthe scrap resin into a first inverted cyclone feed hopper having athroat outlet which directly communicates with the first feed inlet ofthe extruder.
 11. The process of claim 2 wherein the scrap thermoplasticresin to be ground into fluffy, low-density, scrap resin is derived fromthin-film scrap resin.
 12. The process of claim 2 which includesconveying the fluffy ground scrap resin after grinding to a first feedhopper by blowing air to convey the fluffy scrap resin through a tubeand into the first feed hopper, and feeding the fluffy resin by gravityfrom the first feed hopper to the first feed inlet.
 13. The process ofclaim 2 wherein a throat area of the first feed inlet is from about twoto six times a throat area of the second feed inlet.
 14. The process ofclaim 2 which includes feeding the scrap resin in combination with agranular virgin resin pellets into a first feed inlet, and filling about40 to 70% of the extruder barrel volume with the mixture.
 15. Theprocess of claim 2 wherein the density of the high-density resin is fromabout 3 to 10 times greater that that of the low-density scrap resin.16. The process of claim 2 wherein the second and first feed inlets arespaced apart by a distance of from about one to three lead screwlengths.
 17. The process of claim 2 wherein the fluffy scrap and pelletsresin has an average particle size ranging from about 1/16th to 1/4th aninch.
 18. The process of claim 2 wherein the resin pellets are a virginthermoplastic resin in pellet form.
 19. The process of claim 2 whereinthe extruder product when the first feed inlet is not plugged by thescrap resin comprises more than about 10% by weight of scrap resin. 20.The process of claim 2 wherein the fluffy scrap and pellet resin are fedinto the respective feed inlets by gravity.
 21. A process for the use ofa high amount of fluffy scrap polyethylene resin in an extruder, whichprocess consists essentially of:a. grinding salvageable polyethyleneresin film into a fine particle-size, fluffy, polyethylene resin scrapof low density of a character having a tendency to block an extruderbarrel feed inlet; b. mixing the ground polyethylene resin scrap withgranular virgin polyethylene resin to obtain an admixture ofscrap-to-virgin resin having a ratio of from about 1:1 to 6:1; c.conveying the admixture into a first cyclone-type feed hopper having athroat outlet; d. feeding the admixture from the cyclone hopperautomatically by gravity or low-pressure difference through the throatoutlet directly into a first feed inlet in a barrel of a screw-typeextruder, the amount of the admixture fed into the barrel of theextruder being insufficient to fill the barrel, and which admixturetends occasionally to plug the throat outlet of the cyclone hopper: e.feeding a virgin granular polyethylene resin by gravity from a secondauxiliary feed hopper into a second feed inlet in the tube of theextruder, the second feed inlet downstream of and positioned about oneto three helical screw lengths from the first feed inlet, the virginmaterial fed in an amount sufficient to keep the barrel automaticallyfilled to the extruder die with the resin at all times when the throatoutlet is plugged with the admixture, so that irregular blockage in thethroat area of the first feed hopper by the fluffy scrap resin will notresin-starve the extruder die, the throat area of the first feed inletbeing larger in throat diameter than the throat area of the second feedinlet and second throat area from the second feed hopper; and f.compressing, heating and extruding the admixture and the polyethylenevirgin resin to produce continuously an extruded product, said productcontaining a high percentage of scrap polyethylene resin when the firstfeed inlet is not plugged.
 22. The process of claim 21 which includes:a.grinding salvagable polyethylene resin in film form into fluffypolyethylene resin scrap of low density; and b. conveying an admixtureof scrap resin and virgin pellet resin after grinding and mixing to afirst feed hopper by blowing air through a tube which conveys theadmixture into the first feed hopper, and by gravity to the first feedinlet.